You are placing more importance on the distinctions than any denizen of the ancient city would. The simple answer is that nobody cared whether a person was slave, freed slave, or just some low-class (but free) schmuck. They generally wore the same clothes and moved through their lives in a similar fashion, regardless of their specific status. Free, but poor, Roman citizens went to vote, and had legal rights not afforded to slaves, but neither of those were every-day. Roman citizens could not be beaten in the streets, for instance, and slaves could, but street beatings were not happening every twenty minutes in the wards of the city. If an arrogant Roman governor decided someone was in his way and ordered his lictors to beat that unfortunate soul, the question of the victim's status would not be sorted out in that moment. If it was ever sorted at all, it would happen later, in the courts. So for instance, the governor Verres had a Roman citizen flogged and (essentially) robbed while governor in Sicily, and that citizen later organized a legal defense for himself. That complaint, lumped in with other actions, went all the way to Rome in the capable hands of a young lawyer named Cicero. But in the moment, he was flogged and all his protestations of citizenship was meaningless. Slaves who were trusted enough to wander around typically stayed in the local neighborhood, where everyone knew them and knew their status, and their master, and everything else. Slaves did not wander from the Aventine to the Suburra to buy themselves a doughnut, and a low-class stranger idling about in a neighborhood he did not live in was suspicious regardless of his legal status. Conversely, the tutor of a high-class Roman child, even though a slave, would treat a rude low-class citizen with a certain amount of haughtiness, despite their relative status. A Roman consul's slave tutor was vastly more important than a free citizen cart-drover in any given situation on the street.

The very top tier of Roman society was obvious: the important Romans strutted around in the toga and were followed by an entire retinue. The more people (clientes) you had in your posse, the more important you were. If a Roman senator decided to go to the fish market by himself, wearing the "street clothes" of a typical low-class laborer, no one would look at him twice. Conversely, if a slave were somehow able to put on a purple-banded toga and convince twenty other toga-clad persons to follow him around in the forum, he would be accepted as a high-status person (though many would wonder who he was and immediately try to find out). He would probably last about 5 minutes before someone came up to him to ask who he is, who his father is, where he is from, and why he'd never been seen before.

About the forehead brands: my sense is that this is not something you'd see every day as you walked around. Runaway slaves were typically not the "urban" type--they ran from hard labor in the mines, or on industrial-scale farms, where life was very tough and worth the risk of flight. A slave living in a master's urban residence typically had a really good life, even compared to a lot of free but poor citizens, and even if they had an opportunity or a plan, running away would not make sense to them. Where would they go? How? And do what when they got there? I don't have a good source for this at my fingertips, though, and I wonder if we have actual demographic data on runaway slaves: where, who, when, why. Maybe /u/xenophontheathenian or /u/legalaction or /u/iphikrates has an idea for a good source on this question. I can't find anything relevant in the typical "Roman Daily Life" sources I have nearby.

update for those who might read later: I found a little bit of discussion about this topic by Neville Morley, "Social Structures and Demography" in the excellent Blackwell Companion entry edited by Rosenstein and Morstein-Marx:

The ideological distinction between freedom and slavery was clear; it is less certain how far it worked in practice. Indeed, it has been argued that the Roman elite developed the ‘‘legal fiction’’ of absolute dominion of masters over slaves in response to the prevalence of slavery and the difficulty in distinguishing free from slave, since there was no clear-cut distinction in dress or occupation.7 Slaves might at times have an advantage over free men; their value to their owners meant that they would be fed and clothed, whereas the independent freeborn might be left to starve. Slaves could sometimes possess a sort of power, since those who served as personal attendants became so intimate with their owners; certainly the elite had far greater and more regular contact with such slaves than with the poor masses. Slaves were not treated identically; there was a clear hierarchy of status, from the chain gang in the fields to intimate body servants and secretaries. Some slaves were far better educated than the vast majority of the population, and there was even the possibility of intimate and affectionate relationships, such as that between Cicero and Tiro, which were free from constant competitive element of friendships with ‘‘equals.’’ Manumission was a reasonable prospect for some categories of slave, who might then rise to prominence amongst the plebs, or in a few cases even higher in society, despite the taint of slavery. In some later sources, such as Juvenal’s satires, there is anger and bewilderment that slaves, let alone ex-slaves, might have a higher status than freeborn Romans because of their wealth and influence. Whatever the ideological assumption that ‘‘slave’’ equates with ‘‘dependent’’ and is always inferior to ‘‘free,’’ in practice the situation seems sometimes to have been much more complicated.

There is also the fundamental problem of knowing how the mass of the population thought about social status; whether they valued citizenship as highly as the politicsobsessed elite who produced the written sources, and whether they thought in terms of the free – slave divide. We have no useful accounts of social relationships from either the free poor or slaves. The slave characters in the plays of Plautus, like the depiction of the cunning slave in the anonymous Life of Aesop, offer insight into the anxieties and curiosity of the elite about the ‘‘itchy eye’’ – annoying but indispensable – of slavery, while Apuleius’ powerful fable of the plight of the slave, desperately attempting to prove himself human, is in the end equally limited, a work of imagination rather than experience (see also Chapter 25).8 Epigraphical evidence gets us closer to the attitudes of the ordinary members of society who commissioned inscriptions, but it is of course a matter of public display rather than private thoughts, with all the limitations that that implies, and is biased towards those with the wealth and motivation to put up inscriptions, not a representative sample of Roman society as a whole.9 That ‘‘epigraphic habit’’ does constitute evidence of status concerns in its own right; epigraphy shows that many freedmen did wish to emphasize and advertise their achievement of freedom, to the extent that their presence in the population of Rome, and the frequency of manumission, has often been overestimated. It can also be noted that these freedmen, and the others who declined to record their legal status, did not accept the elite valuation of their occupations in status terms; plenty chose to identify themselves as craftsmen and traders, basing their public identity on their profession. We might speculate that wealth and lifestyle – which floor of the apartment block they lived on, variety and security of diet, level of education, officeholding in an association, funeral arrangements (from mass burial pits to a place in a communal mausoleum to an elaborate, self-advertising tomb) – were far more important to most Romans than strict legal or political status.10 The most obvious problem with employing the idea of status is that most of our impressions of its operation come from the elite, who were obsessed with the struggle for prestige. It is virtually an ‘‘actors’ category,’’ and as such subject to the limitations noted earlier for Cicero’s view of society. All too easily it supports a view of an ordered, hierarchical society in which everyone knew his or her place and knew the proper way to behave toward those in other status groups; whereas, as has been suggested here, recognition of the status of others and assertion of one’s own social standing was always a matter of negotiation and even argument, with disputes over which status criteria (wealth, birth, or lifestyle, for example) should be applied. Other serious objections are that an approach based on status groups tends to be descriptive rather than analytical – the groups remain more or less fixed, even as individuals move between them – and that it focuses attention too much on individuals and their own sense of social identity, ignoring more significant structures that shaped social action without the participants being wholly aware of them. Disputes over lifestyle choices and personal identity offer little basis for collective action or social conflict; in which case one must wonder why conflict nevertheless takes place.